Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
Referring to FIG. 1, a diagram of a conventional STT-MRAM cell 100 is illustrated. The STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ) storage element 105, transistor 110, bit line 120 and word line 130. The MTJ storage element is formed, for example, from a pinned layer and a free layer, each of which holds a magnetic field in an example, separated by an insulating (tunnel barrier) layer as illustrated in FIG. 1. The STT-MRAM bit cell 100 also includes a source line 140, sense amplifier 150, read/write circuitry 160 and bit line reference 170. Those skilled in the art will appreciate the operation and construction of the memory cell 100 is known in the art. Additional details are provided, for example, in M. Hosomi, et al., A Novel Nonvolatile Memory with Spin Transfer Torque Magnetoresistive Magnetization Switching: Spin-RAM, proceedings of IEDM conference (2005), which is incorporated herein by reference in its entirety.
Conventional magnetic tunnel junction (MTJ) bit cell designs are non-symmetric. That is, the MTJ and hexagonal bottom electrode (BE) plate are not symmetric along the center line of the poly silicon. The MTJ seed, contact, and vias are also not centered. In an example, the non-symmetric design of conventional MTJ bit cells makes it more difficult to design MTJ array structures and/or less area efficient. In an example, the non-symmetric design of conventional MTJ bit cells is a limiting factor in reducing bit cell size. In an example, the non-symmetric design of conventional MTJ bit cells also potentially increases the mismatch in the MTJ pairs, especially when a source line (SL) is parallel to a bit line.